Sapphire is a precious gemstone which occurs as a transparent variety
of the mineral corundum. The ruby is a red variety of gem-quality corundum
and although the term "sapphire" is often applied to gem-quality varieties
of a corundum of all other colors, the traditional sapphire is deep blue,
the best tint being a corn-flower blue. The various colours in corundum are
caused by small amounts of impurities, such as chromium, iron, or titanium,
in the aluminum oxide of which the mineral is essentially composed. Some
sapphires exhibit asterism when cut cabochon (in convex form and not faceted)
and are highly valued star sapphires.
